An example of a solid-state image pickup unit is a CMOS (Complementary Metal Oxide Semicondoctor) type image sensor that reads a photocharge accumulated in a pn junction capacity of a photodiode as a photoelectric conversion device through a MOS transistor.
In this CMOS type image sensor, an operation of reading a photocharge accumulated in the photodiode is executed on each pixel, each row, or the like. Therefore, exposure periods in which the photocharge is accumulated in all pixels are not allowed to coincide with one another, and in a case where a subject is moving, or the like, distortion occurs during image pickup.
Therefore, global exposure in which image pickup is performed on all pixels in a same exposure time is executed.
Then, as one method of achieving global exposure, a mechanical shutter system using a mechanical light-shielding means is widely used.
In the mechanical shutter system, the exposure periods in all pixels coincide with one another by mechanically opening and closing a mechanical shutter. Then, signals are sequentially read after the mechanical shutter is closed to bring about a state in which a photocharge is not substantially accumulated.
However, in the mechanical shutter system, a mechanical light-shielding means is necessary; therefore, downsizing is difficult. Moreover, mechanical driving speed has its limit; therefore, simultaneity is lower than that in an electrical method.
Moreover, a global shutter system in which all pixels are driven simultaneously by electrical control without using the mechanical shutter system to achieve global exposure is also used. More specifically, accumulation of signal charges in an entire pixel array starts simultaneously by simultaneously performing reset driving on photodiodes of the pixel array in all pixel rows. Then, the accumulation of the signal charges in the entire pixel array is simultaneously terminated by simultaneously performing transfer driving to charge accumulation sections such as floating diffusions in all pixel rows.
It is to be noted that reading of the signal charges accumulated in the charge accumulation sections is performed by row-sequential scanning.
In a case where this global shutter system is adopted, it is necessary to provide a light-shielding film or the like above the charge accumulation section. When light enters the charge accumulation section in a period in which signals are sequentially read, light is added to the signals as noise; therefore, the light-shielding film or the like is provided to prevent this.
However, in a case where the light-shielding film is provided, an opening area of the photodiode is reduced to cause a reduction in sensitivity and a reduction in saturation sensitivity. Moreover, since the charge accumulation section is provided in a position relatively close to the photodiode where light enters in a lateral direction, during signal reading, light may be leaked by a light diffraction phenomenon, a light scattering phenomenon, or the like to enter the accumulation section, thereby causing an increase in noise. As results of the reduction in saturation sensitivity and the increase in noise, image quality degradation is caused.
Further, when the charge accumulation section is configured of a floating diffusion layer formed on a silicon substrate, a dark current is easily generated by a crystal defect at an interface between the silicon substrate and an oxide film; however, when a charge is held in the floating diffusion layer, a difference in the dark current applied to a signal level is caused by order of reading pixels. This difference in the dark current is not allowed to be cancelled by noise removal by a reset level.
As a technique of solving an issue that the above-described difference in the dark current is not allowed to be removed, a configuration in which a memory section that accumulates a charge is included in addition to the floating diffusion layer in the pixel has been proposed (for example, refer to PTL 1 and PTL 2). The memory section temporarily holds a photocharge accumulated in the photodiode. A transfer gate is provided to transfer the photocharge accumulated in the photodiode to the memory section.
However, in such a configuration in which the memory section is provided, it is necessary to also shield the memory section from light, and a light-shielded area with respect to a pixel area is increased, and the opening area is further reduced; therefore, a further reduction in sensitivity is caused, and saturation sensitivity is reduced.
On the other hand, a means to expand the opening area of the pixel with respect to incident light is a backside illumination type solid-state image pickup unit. In the backside illumination type solid-state image pickup unit, a circuit configured of a transistor, a wiring line, and the like is formed on a substrate surface (a front surface) opposite to a light-incident surface (a back surface); therefore, since there is an advantage that a large opening of the pixel on a light incident side is secured, miniaturization is made possible.
Incidentally, in a case where a global shutter function is added to the backside illumination type solid-state image pickup unit, to prevent entry of light into a front surface side of a semiconductor substrate, a configuration in which a light-shielding film is formed on a light incident side is adopted. In this case, when a large light-shielding film is formed, the opening area of the pixel is reduced, thereby resulting in difficulty in miniaturization.
As a means to eliminate this, a configuration in which a capacitor is provided outside the silicon substrate has been proposed (for example, refer to PTL 3). However, in this configuration, a dark current generated by the capacitor is large; therefore, high image quality is not allowed to be obtained.
Furthermore, there has been proposed a configuration in which a photoelectric conversion film with a high absorption coefficient is used for a photoelectric conversion film of the backside illumination type solid-state image pickup unit to double as a light-shielding film, thereby having a global shutter function (for example, refer to PTL 4).
In this configuration, entry of light into the accumulation section is allowed to be prevented from causing smear noise by providing the photoelectric conversion film on the light incident side of the substrate.
This configuration includes a first accumulation section that temporarily holds a charge generated by the photoelectric conversion film and a second accumulation section for holding signals in a period in which signals are sequentially read after exposure.